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b=FX8knPJ1LaHX9Ho0qJKmvnxjo+Igu6cLNBzpWbLKrTEFe3vezWGNR1UM5VbpFrxW5 1e3cn7biJterKSZn1/Hui8CaIMJNtkh/ZvZf9PZxHVp0OKwDNTAYlCHjcrh+uRwsB7 dfeAb5PrdA6ukCZt968+3smf2GmBpDBbBCFkN9tQaIgI8x0h4RBxwPPkOUikGqwLTU onL3GS4uIWzo8CU4MvhXzbPP/KCXvLPb+eDG1LpllpwI81W5whFjp8GzEAd9NeGS9P GKshtCatVRRc0iWmQB76a4EUwxrXNw3bJ4GI3RPG0HXjMwg/q9mRMIJ8rrxSmc1Uhh vIMQxnKU6vc7A== Date: Fri, 3 Jul 2026 11:11:14 +0100 From: Lorenzo Stoakes To: "David Hildenbrand (Arm)" Cc: Mike Rapoport , Andrew Morton , Linus Torvalds , Alexander Viro , Christian Brauner , Jan Kara , Oleg Nesterov , Peter Xu , vova tokarev , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org Subject: Re: [PATCH] userfaultfd: prevent registration of special VMAs Message-ID: References: <20260617194059.2529406-1-rppt@kernel.org> <5a993689-f730-406d-8515-8bb6025cc851@kernel.org> <41ef0dce-e973-4947-b5e3-150fdb07f1a6@kernel.org> <11bae87d-73e6-4946-a41f-c5542fb40b75@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <11bae87d-73e6-4946-a41f-c5542fb40b75@kernel.org> X-Rspamd-Server: rspam08 X-Rspam-User: X-Stat-Signature: asei1ixd6odrqdjw16gt3z91tasaywkg X-Rspamd-Queue-Id: 2E0E91C0002 X-HE-Tag: 1783073482-172666 X-HE-Meta: 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 hPAOK9Qg p9MS7tmp8WEdRrydQbk5RWpC+T+AaOdieNoTzIuqK/IukQz0fCo5A1FkqMbG17XuVrVcbR0aXjKAeu75N0OCRZaVXKE8qzED339uBxciIuZZ21ulcsPkCB+Krm/yxOAbvZsGzW0SWByLum8lHkRr1cdfQgJWK3H6m5PPg7cEaEsf6gE8xmIcT95HcjHlWRGzoMpDkLK7afMRWpjgalYOx9CvglgzBFw9xci8H0Djq3+GtRDKaAEfEwm8y78HVBvkoysTj Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Jun 18, 2026 at 11:37:10AM +0200, David Hildenbrand (Arm) wrote: > On 6/18/26 11:35, Mike Rapoport wrote: > > On Thu, Jun 18, 2026 at 11:25:31AM +0200, David Hildenbrand (Arm) wrote: > >> On 6/18/26 11:21, Mike Rapoport wrote: > >>> > >>> Cleaner in what sense? > >>> Will be uglier for sure, just take a look at vma_can_userfault(). > >> > >> I was thinking of this: > >> > >> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > >> index 180bad42fc79..8a6803618a91 100644 > >> --- a/mm/userfaultfd.c > >> +++ b/mm/userfaultfd.c > >> @@ -2029,7 +2029,10 @@ bool vma_can_userfault(struct vm_area_struct *vma, > >> vm_flags_t vm_flags, > >> { > >> const struct vm_uffd_ops *ops = vma_uffd_ops(vma); > >> > >> - if (vma->vm_flags & VM_DROPPABLE) > >> + if (vma->vm_flags & (VM_DROPPABLE | VM_SHADOW_STACK)) > >> + return false; > >> + > >> + if (!is_vm_hugetlb_page(vma) && (vma->vm_flags & VM_SPECIAL)) > >> return false; This is still pretty gross, and it's a bit odd to me that we will now enforce userfaultfd on ranges that are somehow VMA_DONTEXPAND_BIT but not otherwise 'special'. But I think that's because we even allow that to exist as a thing and have made VMA_DONTEXPAND_BIT a little unclear in its behaviour. But I'm fixing that, I'm working on a series that clears up the special flags, and replaces the checks in general with vma_xxx() or vma_flags_xxx() predicated, and which will ultimatately drop VM_SPECIAL/VMA_SPECIAL_FLAGS. > > > > In a way that's an extra check for hugetlb, but it will work. > > My point would be that we exclude all special VMAs, except hugetlb (which is > special but supported ... in its special way). Mike - you said you were respinning, it'd help my series if you respan the above quickly so I could base my change on that :). I can send a quick patch if you're tied up also! > > -- > Cheers, > > David > Thanks, Lorenzo